A storage system provides a logical volume, which is a logical storage area, to a higher-level apparatus, such as a host computer. In addition to a normal volume to which a storage system physical storage area of the same capacity as the logical volume is allocated beforehand, the logical volume includes a Thin provisioning volume to which a physical storage area of the required capacity is dynamically allocated to the logical volume when a write has been performed. In a Thin provisioning volume, a large capacity virtual volume is provided to the host computer, and when a write request has been received from the host computer, a physical storage area inside the storage system is allocated only to the area corresponding to the write. Therefore, compared to the normal volume, the Thin provisioning volume makes it possible to reduce the physical storage area used by the storage system, and, in turn, to greatly reduce costs.
Patent Literature 1 discloses technology for migrating normal volume data to a Thin provisioning volume. In accordance with this, the storage system exercises control such that a physical storage area inside the storage system is allocated once to the entire area of the migration-destination Thin provisioning volume, and thereafter, a physical storage area inside the storage system is allocated only to the area in which the user data is stored, thereby reducing physical storage area consumption and enabling the effective use of the physical storage area.